(1) Field of the Invention
The present invention relates to a novel disazo compound, a method of making same and an electrophotographic element, in particular a multilayer type electrophotographic element which comprises a charge carrier generating layer containing a substance capable of generating charge carriers when exposed to radiation of light (which will be called a charge carrier generating substance hereinafter) and a charge transfer layer containing a substance capable of accepting charge carriers generated from said charge carrier generating layer and transferring them (which will be called a charge transfer layer hereinafter).
(2) Description of the Prior Art
As electrophotographic elements there have typically been used inorganic ones using selenium and its alloys or dispersed coloring matter-sensitized zinc oxides in a binder resin, and organic ones using charge transfer complexes of 2,4,7-trinitro-9-fluorenone (which will be called TNF hereinafter) and poly-N-vinylcarbazole (which will be called PVK hereinafter) and the like. However, the fact is that these elements have a number of advantages as well as various disadvantages. For instance, the now widely used selenium element is defective in that the manufacturing conditions are complicated, the production cost is expensive, it is difficult to form into a belt-like shape due to lack of flexibility, and attention must be paid in handling because it is very sensitive to heat and mechanical impact. The zinc oxide element is inexpensive because it can be prepared by coating the substrate with a cheap zinc oxide, but defective in that it is generally inferior in sensitivity and mechanical properties such as surface smoothness, tensile strength, friction resistance and the like. Therefore, the zinc oxide element involves various problems such as durability and the like when used repeatedly in plain paper copying machines. And, the element using the charge transfer complex of TNF and PVK is too low in sensitivity to be suitably used in high-speed copying machines.
In recent years, a wide range of studies has been carried out in order to eliminate the shortcomings inherent in these elements. In particular, various organic elements have been proposed for that purpose. Among them, multilayer type elements are attracting public attention as elements for use in plain paper copying machines due to their high sensitivity and stable chargeability as compared with usual organic elements, said multilayer type element comprising an electrically conductive substrate; a thin charge carrier generating layer formed on said substrate by depositing a charge carrier generating pigment capable of generating charge carriers on exposure to light in the proper way, for instance, such as vacuum vapor-deposition, coating a pigment solution or coating a dispersion of fine powdery pigment in a resin solution; and a charge transfer layer, formed thereon, in which the charge carriers generated by the charge carrier generating layer can be injected with efficiency and transferred. And, some of them are put to practical use.
As the conventional multilayer type elements of this sort there are known
(1) the multilayer type element using, as the charge carrier generating layer, a thin layer formed by vacuum-vapordepositing a perylene derivative and incorporating an oxadiazole derivative in the charge transfer layer (which see U.S. Pat. No. 3,871,882),
(2) the multilayer type element using, as the charge carrier generating layer, a thin layer formed by coating an organic amine solution of Chloro Dian Blue and incorporating a hydrazone compound in the charge transfer layer (which see Japanese Patent Publication No. 42380/1980),
(3) the multilayer type element using, as the charge carrier generating layer, a thin layer formed by coating an organic solvent dispersion of distyrylbenzene type disazo compound and incorporating a hydrazone compound in the charge transfer layer (which see Japanese Laid Open Patent Application No. 84943/1980), and the like.
However, the fact is that even in the multilayer type elements of this sort the conventional ones have a number of advantages as well as various disadvantages.
The element using the perylene and oxadiazole derivatives disclosed in the preceding (1) involves the problems that the cost of production is raised because the charge carrier generating layer is formed by vacuum vapordeposition, and the sensitivity is insufficient to be used in the higher speed copying machines even if there is caused no problem when used for practical purpose.
The element using the Chloro Dian Blue and hydrazone compound disclosed in the preceding (2) is superior in sensitivity but involves many disadvantages in the preparation because there is necessity of using a hard-to-handle organic amine (for instance, ethylenediamine) as a coating solvent for the formation of the charge carrier generating layer. Further, this element is inferior in the reproductivity of red images from the original because its sensitivity visible light wavelengths covers the range of about 450 to 660 nm. This necessitates employing a filter to filter out red light when setting this element actually in the copying machine, thereby exerting an unprofitable-influence upon the copying machine design.
The element using the distyrylbenzene type disazo compound and hydrazone compound disclosed in the preceding (3) is very profitable in the preparation because the charge carrier generating layer can be formed readily by coating a dispersion of disazo compound, but is defective, like the element disclosed in the preceding (2), in that the reproductivity of red images from the original is inferior because its sensitive light wavelengths cover the range of about 450 to 700 nm.
As the disazo compounds used in the multilayer type elements there are also known for instance the benzidine type disazo compound disclosed in Japanese Laid Open Patent Application Nos. 37543/1972 and 55643/1977, the stilbene type disazo compound disclosed in Japanese Laid Open Patent Application No. 8832/1977 and the like.
However, the multilayer type elements using these conventional disazo compounds were generally inferior in sensitivity and deteriorated in the reproductivity of red images from the original because their sensitive light wavelengths cover the range of about 450 to 700 nm. Accordingly, these elements exerted an unprofitable influence upon the copying machine design because the above disadvantages necessitate employing a filter to filter red light when setting these elements actually in the copying machines.